Computerized systems all around us are recording and saving information about personal behavior: e.g., browsing on the internet, financial transactions linked to customer loyalty accounts, electronic road tolls, use of electrical power in homes and businesses. The trend is for even more information to be gathered, e.g., as radio frequency identity (RFID) tags become commonplace on commercial goods. These trends raise questions: What personal information derivable from such data can be viewed as "private?" What legal and technical means can be brought to bear to protect that privacy?